Opioid Agonist Treatment, or OAT, refers to the treatment of a narcoticsaddiction in humans via the administration of similar opioid drugs, agonists, and the resultant cross tolerance and physical dependence. Methadone and Buprenorphine are the most common drugs used in agonist treatment. Only facilities accredited by CSAT are able to provide this service.

The use of agonist drugs to treat a narcotics addiction is tightly regulated in the United States. Diversion, or potential diversion, of agonist drugs is often the domain of the Drug Enforcement Administration, as they are defined as criminal acts.